This invention relates to induction motors and more particularly to an induction motor having a rotor capable of reducing operating loss due to distorted alternating current when driven by a non-sinusoidal power supply such as a power supply including inverters and the like.
Generally, from the standpoint of the difference in the rotor structure, induction motors are classified into three major types, namely, a wound-rotor induction motor having a rotor core formed with slots in which rotor windings are seated, a squirrel-cage induction motor having a similar rotor core formed with slots in which conductors put together in a squirrel-cage configuration are seated, and a solid rotor induction motor having a rotor core which acts by itself as a winding or which has conductors secured to its overall surface. Among these induction motors, the squirrel-cage induction motor having a squirrel-cage rotor of excellent electrical characteristics which is robust and relatively inexpensive is typical and used in most fields.
In the squirrel-cage rotor, a rotor core of laminated thin iron sheets is fixedly secured to a rotary shaft. Grooved in the circumferential surface of the rotor are a plurality of slots which extend axially and open to the surface, the slots being spaced circumferentially from each other by an identical distance. Conductor bars of copper or aluminum are then inserted in the slots and short-circuited by short-circuiting rings as applied to opposite ends of the rotor. The squirrel-cage rotor of the above structure is inserted into a stator and supported therein with an air gap therebetween to complete the induction motor. In the induction motor, rotating field generated in the stator induces electromotive forces in the conductor bars, thus causing current flow in the conductor bars. The current flow interacts with the rotating field to rotate the rotor. When the induction motor is driven approximately under ratings with a typical sine-wave power supply, magnetic flux incident upon the rotor has a very small frequency of several Hz so that the current flow in the conductor bars is uniform and hence the squirrel-cage rotor can be operated with safety under stable thermal conditions.
Currently, however, a variable frequency power supply including thyristor inverters and the like has been used for speed control of the squirrel-cage induction motor. The output of the variable frequency power supply is in general a distorted alternating current which contains harmonics components. Accordingly, the magnetic flux generated in the stator contains a variety of time-variant harmonics and, in the squirrel-cage rotor in operation, additional loss due to the harmonics components is caused. Disadvantageously, loss in the squirrel-cage rotor is considerably increased when the motor is driven by the distorted waveform as compared when driven by sine-wave alternating current.
One may refer to IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS (Vol. PAS-86, No. 7 JULY 1967) "Polyphase Induction Machine with Slitted Ferromagnetic Rotor: I--Experimental Investigations and a Novel Slipmeter" disclosing an induction motor with a slitted rotor. This publication simply describes change in the position of slits and corresponding characteristics of the induction motor.